Light-emitting diode device

ABSTRACT

A LED device which has an excellent color rendering property and no toxicity and does not bring about an increase in production cost more than necessary, and a covering member used for such a LED device can be provided.  
     The white LED device comprises a light-emitting element for emitting light of blue to green region and a fluorescent substance containing a red phosphor for converting a wavelength of the light emitted from the light-emitting element. The red phosphor is CaS or activated by Eu or a phosphor expressed by a general formula AEu (1-x) Ln x B 2 O 8  wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo; and x is number equal to or larger than 0, but smaller than 1.

FIELD OF THE INVENTION

[0001] The present invention relates to a white light-emitting diode device having an excellent color rendering property which is used in a back light and a front light as an externally irradiating light source of a color liquid crystal display and used for room illumination and, particularly to a covering member enabling a conventional blue or green light-emitting diode device to emit white light, and a white light-emitting diode device using the covering member.

DESCRIPTION OF THE RELATED ART

[0002] A light-emitting diode (hereinafter, referred to as “LED”) emits a light when electrons from an N-region and holes from a P-region are moved to a PN junction section and recoupled in a PN junction semiconductor when a forward voltage is applied. Since energies released when the free electrons are coupled are radiated as light, the light emitted from the LED is light in a narrow wavelength range, i.e. light of a single color such as red and blue.

[0003] On the other hand, a light source for illumination of a color liquid crystal display and for room illumination needs to emit fluorescent light or white light approximate to natural light.

[0004] Thus, a full-color LED device has been used as a light source for illumination of the color liquid crystal display and for room illumination. The full-color LED device generates white light by using a combination of a red light-emitting element for emitting red light, a blue light-emitting element for emitting blue light and a green light-emitting element for emitting green light.

[0005] However, using a full-color LED device as a light source causes a complicated design, since each of the light-emitting elements in the full-color LED device requires an anode terminal and a cathode terminal. Further, a larger space and higher costs are required since a full-color LED device uses three kinds of light-emitting elements. In addition, if one light-emitting element of a full-color LED device is broken, a desired color balance is lost, with the result that an intended color tone may not be obtained. In view of the above situation, many studies are being vigorously made on the LED device capable of emitting white color using one kind of light-emitting element. Such an LED device is called a white LED device and distinguished from the full-color LED device. Using the white LED device enables a back light or the like to be driven by a simple electric circuit without requiring an inverter circuit and the like, and performs reduction of power consumption. This presents advantages of reducing the outer configuration of a driving circuit and eliminating an electromagnetic noise. Due to such excellent advantages, the white LED device is being developed.

[0006] Since a white LED device must generate white light only by using one kind of light-emitting element (single-color), it is a general practice to use a single-color light emitting element in combination with a phosphor which can convert the wavelength of a light emitted from the light-emitting element to emit a light of an other color. For example, white LED devices in which a layer containing a YAG (yttrium aluminate) phosphor is provided on a blue light-emitting element are commercially available.

[0007] The YAG phosphor generally emits light of green to yellow region depending on the kind of an activating element in the YAG. Thus, red light is insufficient in the white LED obtained by combining a blue light-emitting element for emitting light having an excitation wavelength of YAG and the YAG phosphor. This leads to a problem that a matter to be displayed in red looks subdued.

[0008] It may be considered to use YAG capable of emitting fluorescent light approximate to red light by devising the kind of the element to activate YAG, for example, by substituting Gd for Y-site of Y₃Al₅O₁₂ to shift an emission peak toward a longer wavelength side, i.e. red side. However, such a method cannot provide a white LED device having an average color rendering evaluation number (Ra) of 85 or larger which is an index as to whether an object being illuminated conforms to color perception under a reference light. Although more red light may be obtained by increasing the content of red YAG in the fluorescent layer, the emission intensity of yellowish light emitted from YAG also becomes higher, with the result that the average color rendering evaluation number Ra of 85 or larger cannot be obtained.

[0009] As a method for compensating for not only the blue, green and yellow lights, but also the red light in the white LED, it may be considered to use a red phosphor. However, a manganese activated magnesium arsenate phosphor known as a dark red phosphor for a fluorescent lamp is not presently being produced due to its toxicity. Further, since manganese activated magnesium fluoride germanium oxide phosphor is expensive, it is not suitable for the application of the white LED as a back light which has merits of being light-weight and thin and requiring a low power consumption.

[0010] Adding of a red pigment into a fluorescent layer is also being studied (see, for example, Japanese Unexamined Patent Publication No. (HEI)5-152609). However, since the pigment is like a light filter for making a specified color stand out by cutting other colors, addition of the pigment results in a reduced luminance of an LED device and, thus, is not suitable for the white LED device aiming at a smaller space, lower costs and power saving.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide an LED device having an excellent color rendering property without increasing manufacturing costs more than necessary and a covering member used for such a LED device.

[0012] An inventive light-emitting diode comprises a light-emitting element for emitting light of blue to green region and a fluorescent substance containing a CaS phosphor activated by Eu for converting a wavelength of the light emitted from the light-emitting element.

[0013] Another inventive light-emitting diode device comprises a light-emitting element for emitting light of blue to green region and a fluorescent substance containing a phosphor expressed by a general formula AEu_((1-x))Ln_(x)B₂O₈ for converting a wavelength of the light emitted from the light-emitting element, wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo; and x is number equal to or larger than 0, but smaller than 1.

[0014] An inventive covering member for a light-emitting diode device which includes a light-emitting element sealed by a transparent resin for emitting light of blue to green region comprises a base polymer, and a CaS phosphor activated by Eu.

[0015] Another inventive covering member for covering a light-emitting diode device which includes a light-emitting element sealed by a transparent resin for emitting light of blue to green region comprises a base polymer, and a phosphor expressed by a general formula AEu_((1-x))Ln_(x)B₂O₈ wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo; and x is number equal to or larger than 0, but smaller than 1.

[0016] The inventive light-emitting diode devices are better in emitting red light than conventional light-emitting diode devices using a light-emitting element for emitting light of blue to green region. Accordingly, a white diode device having a color rendering property approximate to that of a fluorescent lamp or a natural light can be provided by containing YAG phosphor in the fluorescent substance.

[0017] Further, if the inventive covering member is mounted on a conventional light-emitting diode device, they can be used as the inventive white LED devices having an excellent color rendering property.

[0018] These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIGS. 1A and 1B are diagrams showing a structure of a main portion of a LED device according to a first embodiment of the present invention, wherein FIG. 1A shows a coating-type LED device and FIG. 1B shows a film-type LED device,

[0020]FIGS. 2A and 2B are diagrams showing a structure of a main portion of a LED device according to a second embodiment of the present invention, wherein FIG. 2A shows a cannonball-shaped LED device and FIG. 2B shows a block-shaped LED device,

[0021]FIG. 3 is a diagram showing a structure of a cap-type LED device according to a third embodiment of the invention,

[0022]FIG. 4 is a diagram showing a structure of a block-type device according to a third embodiment of the invention,

[0023]FIG. 5 is a diagram showing a construction of the LED device using a covering member having an engaging portion in the third embodiment,

[0024]FIG. 6 is a diagram showing a construction of the LED device using a sheet-type covering member in the third embodiment,

[0025]FIG. 7 is a graph showing an excitation spectrum and an emission spectrum of LiEuW₂O₈,

[0026]FIG. 8 is a graph of emission spectra showing a relationship between x in a LiEu_((1-x))Y_(x)W₂O₈ and emission intensity,

[0027]FIG. 9 is a graph showing an excitation spectrum and an emission spectrum of LiEuMo₂O₈,

[0028]FIG. 10 is a graph of emission spectra showing a relationship between the content of a red phosphor and emission intensity,

[0029]FIG. 11 is a graph showing an emission spectrum of a white LED device 4,

[0030]FIG. 12 is a graph showing an emission spectrum of a white LED device 5,

[0031]FIG. 13 is a graph showing an emission spectrum of a white LED device 6, and

[0032]FIG. 14 is a graph showing an emission spectrum of a white LED device 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0033] A light-emitting diode device according to the present invention is characterized by using a light-emitting element for emitting light of blue to green region and a phosphor (hereinafter, referred to as “red phosphor”) capable of converting the light emitted from the light-emitting element into a red light.

[0034] The light-emitting element used in the present invention is a light-emitting element for emitting light of blue to green region, specifically blue light of 400 to 600 nm, preferably 400 to 500 nm, and green light of 500 to 600 nm. This is because the red phosphor used in the present invention is excited by these lights as described later.

[0035] Blue light-emitting elements include light-emitting diodes containing GaN, SiC, or II-IV groups such as ZnS and ZnSe, and green light-emitting elements include light-emitting diodes containing GaP:N or GaP.

[0036] Next, the red phosphor is described.

[0037] The red phosphor used in the LED device according to the present invention is CaS activated by Eu or a compound expressed by a general formula AEu_((1-x))Ln_(x)B₂O₈ wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo.

[0038] CaS activated by Eu is excited by light of 420 to 600 nm and emits light of 570 to 690 nm which peaks at 630 nm.

[0039] AEu_((1-x))Ln_(x)B₂O₈ is a phosphor which emits light near 614 nm by ⁵D₀→⁷F₂ transition of Eu³⁺ ions, wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo. Although an excitation wavelength and an emission wavelength differ depending on the kinds of elements A and B of the phosphor, the red phosphors are commonly excited by light near 470 nm (blue) and or 540 nm (green) and can emit light near 620 nm (red). It should be noted that the excitation and emission wavelengths are same independently of the value of x if the kinds of the elements A and B are same in each of the phosphors.

[0040] In the general formula AEu_((1-x))Ln_(x)B₂O₈ (A=Li, K, Na, Ag; Ln=Y, La, Gd; B=W, Mo), x is equal to or larger than 0 and smaller than 1, preferably is equal to 0. This is because the emission intensity near 615 nm (red) is highest in the case of AEuB₂O₈.

[0041] AEu_((1-x))Ln_(x)B₂O₈ (A=Li, K, Na, Ag; Ln=Y, La, Gd; B=W, Mo) is obtained by mixing oxides, carbonate and the like of elements which constitute the phosphor at a desired stoichiometric ratio.

[0042] By containing the phosphor for emitting red light in the fluorescent substance, a LED device usable as a light source capable of displaying a distinct red color can be provided without using a red light-emitting element.

[0043] In addition to the red phosphor, the fluorescent substance may contain other kind of phosphor for emitting light of a color other than red upon being excited by light from the light-emitting element. It is particularly preferable to contain a phosphor capable of emitting light of the color other than a color light emitted from the light-emitting element and red light. Specifically, an yttrium aluminate phosphor is preferably used.

[0044] The yttrium aluminate phosphor (so-called YAG) is a stable oxide having a garnet structure in which Y-atoms of Y₃Al₅O₁₂ are substituted by Gd at part of their positions, particularly a phosphor which is excited by blue light (400 to 530 nm) to emit light of yellow to green region centering 550 nm. Activating elements to be added to the yttrium aluminate phosphor include cerium, europium, manganese, samarium, terbium, tin, chromium, etc. Among them, cerium is preferable. In other words, Y_(x)Gd_(3-x)Al₅O₁₂ activated by Ce is preferably used. One, two or more kinds of such YAG phosphors may be contained in the fluorecent substance.

[0045] The light emitted from the LED device according to the invention is a combination of the light emitted from the light-emitting element and the light having a wavelength converted by the phosphor. The light derived from the phosphor is combined with the light (blue or green) from the light-emitting element, thereby providing light appearing white. A distinct red color, which has been difficult to be displayed by a conventional white LED device, can be displayed by the inventive LED device due to the red phosphor.

[0046] Further, by containing the phosphor capable of converting the color of the light from the light-emitting element to the one other than red, a white LED device having an excellent color rendering property and capable of realizing various colors can be provided. Specifically, there can be provided the white LED device having an excellent color rendering property: an average color rendering evaluation number Ra of 85 or larger, further a special color rendering evaluation number R₉, which serves as an index for distinct red, of 80 or larger. Such a color rendering property could not be accomplished by changing the kind of YAG, changing the content of a phosphor and adding the pigments in a conventional white LED device.

[0047] The color rendering property is a characteristic of a light source relating to an influence of an illumination light on how the color of an object looks. The higher the color rendering property, the more the color perception of an object under the light source conforms to that of the same object under a specified reference light. The average color rendering evaluation number Ra is an average of color rendering evaluation numbers for specified eight test colors. The higher the color rendering evaluation number, the better the color rendering property for every color.

[0048] A first embodiment of the LED device according to the invention is such that the fluorescent substance has a form of a layer placed on an outer surface of the light-emitting element. For example, a fluorescent layer 2 may be formed by coating the outer surface of the light-emitting element 1 with a coating agent containing a phosphor 4 as shown in FIG. 1A or a fluorescent layer 2′ may be formed by adhering a film made of a resin composition containing the phosphor 4 to the outer surface of the light-emitting element 1 as shown in FIG. 1B. The light-emitting element 1 having the fluorescent layer 2 (or 2′) placed thereon may construct a chip-type LED device by being placed in a cup 3 connected with a lead wire (not shown). The chip-type LED device may be sealed with a transparent resin to provide a sealed-type LED device.

[0049] The coating agent used for forming the fluorescent layer contains the above-mentioned red phosphor, the YAG phosphor if necessary, binder and solvent. The fluorescent layer 2′ is formed by placing a film made from a resin composition comprising the red phosphor and a base resin, and adhering the film to the light-emitting element using the binder or heating the film.

[0050] A second embodiment of the LED device according to the invention is a sealed-type LED such that the light-emitting element is embedded and sealed by a resin containing the red phosphor. One example of the LED device having a cannonball-shape is shown in FIG. 2A. The light-emitting element 1 is placed in the cup 3 connected with a lead frame 8. With this assembly set on a substrate 10 through which the lead frame 8 is insertable, a cannonball-shaped sealed-type LED 11 is formed on the substrate 10 by molding a resin composition containing the red phosphor 4 into a cannonball-shape. The sealed-type LED 11 is formed as to embed the light-emitting element 1 in the fluorescent substance. FIG. 2B shows an example of the LED device having a block-shape. With the light-emitting element 1 placed on a lead wire printed on a substrate 10′, a block-shaped sealed-type LED 12 is formed by molding a resin composition containing the red phosphor 4 into a block-shape.

[0051] The resin composition used for sealing the light-emitting element comprises a transparent resin as a base resin and the red phosphor, i.e. CaS activated by Eu or AEu_((1-x))Ln_(x)B₂O₈ wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo. The base resin may be a transparent resin having a good property of transmitting the light emitted from the light-emitting element and having durability, chemical resistance and insulating property if desired. Specifically, acrylic resin, polycarbonate, polystyrene, polyester, epoxy resin, polypropylene, polyethylene, silicone resin, etc. can be used as the base resin.

[0052] In the resin composition used for sealing the light-emitting element, the content of the red phosphor is preferably 2 to 40 parts by mass with respect to 100 part by mass of the base resin. In the case of containing YAG in the resin composition, a ratio of the content of YAG to that of the red phosphor (YAG: red phosphor) is preferably 1:0.2 to 1:4.

[0053] In the second embodiment, the shape of the sealed-type LED is not limited to the cannonball shape and the block shape. The construction of a terminal to be connected with the light-emitting element is not limited, either.

[0054] In the case that the phosphor is contained in the material used for sealing the light-emitting element as the second embodiment, it is easier to obtain a white LED device having a more stable color tone than the first embodiment by the following reason. A very small amount of the phosphor is contained in the fluorescent layer placed on the outer surface of the light-emitting element since the light-emitting element is very small. Since a variation of the content of the phosphor largely influences the emission characteristics of the LED device, a high application technique and a high quality control of the material including the content of the phosphor are required. On the other hand, since larger quantities of the resin and the phosphor are contained in a sealed-type LED due to the fact that the volume of the sealed-type LED is bigger, the ratio of the content of the resin to that of the phosphor varies only to a small degree. Further, the second embodiment is convenient in view of productivity. In other words, an operation of adhering a film to a small light-emitting element is cumbersome and causes an increase in defect rate. On the other hand, LED devices are generally distributed in the sealed LED on the market. Thus, it is sufficient to add the phosphor to the sealing material in the manufacturing process of the sealed-type LED, and no new processing is required.

[0055] In a third embodiment of the present invention, the light-emitting element is sealed with a transparent resin to form a sealed member, and the sealed member is covered with a covering member including the above-mentioned red phosphor. Specifically, as shown in FIG. 3, a sealed member 20 is formed by sealing the light-emitting element 1 placed in the cup 3 connected with the lead frame 8 and is covered with a covering member 21 containing the phosphor 4. FIG. 4 shows a LED device where the sealed member is block-shaped. A covering member 23 used for covering the block-shaped sealed member 22 is formed so as to conform to the shape of the sealed member 22.

[0056] The shape of the sealed member and a terminal connecting method used in the third embodiment are not particularly limited. For example, as shown in FIG. 5, a sealed member 20′ having a projecting locking portion 24 may be used, thereby accomplishing a stable mounting a covering member 21′ to the sealed member 20′. The covering member 21′ includes an engaging recess 25 engageable with the locking portion 24 of the sealed member 20′.

[0057] Further, as shown in FIG. 6, the side wall of a sealed member 28 in which the light-emitting element 1 is embedded may be surrounded by a reflecting frame 27 to have a rectangular parallelepipedic or cylindrical shape, and a covering member containing the phosphor 4 may be arranged on the upper surface for transmitting the light from the light-emitting element 1.

[0058] The covering member is formed by molding a polymer composition comprising the red phosphor and a base polymer. The base polymer may be a transparent polymer having a good property of transmitting the light emitted from the light-emitting element and having durability, chemical resistance and insulating property if desired. Specifically, acrylic resin, polycarbonate, polystyrene, polyester, epoxy resin, polypropylene, polyethylene, silicone elastomer, polystyrene thermoplastic elastomer, polyolefin thermoplastic elastomer, polyurethane thermoplastic elastomer, etc. may be used as the base polymer. Among these materials, rubbers, elastomers and mixtures of these having an excellent elasticity are preferably used so as to provide a stable covering member which can be easily mounted on the sealed member to cover it and be difficult to detach.

[0059] As a material for the covering member, a preferable composition is such that the content of the red phosphor is 2 to 49 parts by mass with respect to 100 parts by mass of the base polymer. In the case of containing YAG in the covering member, a ratio of the content of YAG to that of the red phosphor (YAG: red phosphor) is preferably 1:0.2 to 1:4.

[0060] The covering member may take any shape which enables it to be mounted on the sealed member and have a light gathering mechanism at its leading end if necessary. Such a mechanism enables luminance to be increased and illumination action to be efficiently performed with a small energy. The light gathering mechanism may be, for example, a lens structure in which thickness is larger at the leading end than at any other part.

[0061] In the case of a cap-shaped covering member, the covering member may be merely mounted on the sealed member. If the sealed member has an engaging portion, it becomes difficult to detach the covering member by engaging the covering member with the engaging portion. In the case of a sheet-shaped covering member, it is preferably secured to the sealed member by adhesive, heat fusion, ultrasonic bonding, or the like.

[0062] In the case of the third embodiment of the present invention, the material for the sealed member does not require to contain the phosphor and can be made of the transparent base resin. This allows a large variety of sealed members to be used and enables color tone of light emitted from a light-emitting element to change easily. In other words, since the resin composition used for forming the sealed member needs to be prepared for at least one lot, the material and the color tone is required to be changed on a large scale, which is not suitable for production of various products in small quantities. In this respect, the third embodiment has an advantage of easier production adjustment since the color tone of the LED device can be changed only by changing the kind of the covering member and, thus, the same material can be used for the sealed member.

EXAMPLES

[0063] [Evaluation Method]

[0064] (1) Excitation Spectrum

[0065] An excitation spectrum (wavelength on horizontal axis, absorbancy on vertical axis) was measured using a spectral fluorescent photometer FP-750 (manufactured by Nippon Bunko K.K.).

[0066] (2) Emission Spectrum

[0067] An emission spectrum (wavelength on horizontal axis, emission intensity on vertical axis) in relation to a specified excitation wavelength was obtained using the spectral fluorescent photometer FP-750 (manufactured by Nippon Bunko K.K.).

[0068] (3) Emission Characteristics

[0069] CIE (International Committee on Illumination) chromaticity coordinates x, y and luminance L of a light diffusedly emitted from the LED device using a spectral radiation luminance meter PR704 (manufactured by Photo Research).

[0070] The higher the luminance, the brighter. The larger the values of the chromaticity coordinates, the more the color is away from white, i.e. a yellowish color.

[0071] (4) Color Rendering Property

[0072] Color rendering property was evaluated by the average color rendering evaluation number Ra and special color rendering evaluation numbers R₉ to R₁₅.

[0073] The average color rendering evaluation number Ra is a color rendering evaluation number corresponding to an average value of the special color rendering evaluation numbers of the specified eight test colors, whereas the special color rendering evaluation number (R_(i)) is a color rendering evaluation number obtained from a change of coordinates in a specified uniform color space when an object is illuminated by a reference light and when it is illuminated by a sample light source.

[0074] [Preparation of the Red Phosphor LiEu_((1-x))Y_(x)W₂O₈]

[0075] Li₂O, Eu₂O₃, Y₂O₃, and WO₃ were weighed to have a stoichimetric ratio and mixed in ethanol in a ball mill. By putting the mixture into an alumina crucible and baking it in an atmosphere of 2000° C. for 2 hours after being dried and crushed, LiEuW₂O₈, LiEu_(0.7)Y_(0.3)W₂O₈, LiEu_(0.5)Y_(0.5)W₂O₈, LiEu_(0.3)Y_(0.7)W₂O₈, and LiEu_(0.1)Y_(0.9)W₂O₈ were obtained.

[0076] A measurement result of an excitation spectrum of the LiEuW₂O₈ is shown in FIG. 7. Existence of excitation bands at 465 nm and 536 nm can be seen from FIG. 7. In other words, LiEu_(x)LnW₂O₈ compound can be found to be effective as a red phosphor for blue and green LED devices.

[0077] Next, emission characteristics of the above prepared phosphors in the case of an excitation wavelength of 470 nm were measured. The obtained emission spectra are shown in FIG. 8. In FIG. 8, horizontal axis and vertical axis represent wavelength (nm) and emission intensity, respectively.

[0078] It can be seen from FIG. 8 that the emission intensity of the LiEu_((1-x))Y_(x)W₂O₈ compound near 620 nm was highest when x=0, i.e. LiEuW₂O₈.

[0079] [Preparation of the Red Phosphor LiEu_((1-x))MoO₈]

[0080] Li₂O, Eu₂O₃, and MoO₃ were weighed to have a stoiohimetric ratio and mixed in ethanol in a ball mill. By putting the mixture into an alumina crucible and baking it in an atmosphere of 600° C. for 3 hours after being dried and crushed, a LiEuMo₂O₈ compound was obtained.

[0081] A measurement result of an excitation spectrum of LiEuMo₂O₈ and an emission spectrum thereof in the case of an excitation wavelength of 470 nm is shown in FIG. 9. In FIG. 9, vertical axis and horizontal axis represent absorbancy or emission intensity and wavelength (nm), respectively, and solid and broken lines represent the excitation spectrum and the emission spectrum, respectively.

[0082] It can be seen from FIG. 9 that a red emission can be obtained at 614 nm by ⁵D₀→⁷F₂ transition of Eu³⁺ ions similar to LiEuW₂O₈ compounds. Further, since the excitation wavelength is peaked at 465 nm and 537 nm, this compound can be used with not only blue LEDs, but also green LEDs.

[0083] [Relationship Between the Content of the Red Phosphor and an Emission Color of the LED Device]

[0084] Compositions for the covering member in which the content of LiEu_(0.7)Y_(0.3)W₂O₈ with respect to 100 parts by mass of silicone rubber were changed as shown in TABLE-1. Using these compositions, the cap-shaped covering members having a thickness of 0.35 mm were molded. These covering members were mounted on a sealed member in which a blue light-emitting element is embedded and a sealed member in which a green light-emitting element is embedded, and the resulting LED devices were turned on at 20 mA in an integrating sphere to measure emission colors. A measurement result is shown in TABLE-1. TABLE 1 Blue LED Chip Green LED Chip Content of Lu- Lu- LiEu_(0,7)Y_(0,3)W₂O₈ mi- mi- (Parts by Mass) X y nance x y nance  0 0.1277 0.0921 24.77 0.2060 0.6075 112.4 40 0.1371 0.1020 18.85 0.2174 0.6676 70.18 80 0.1435 0.1078 14.80 0.2264 0.6629 59.75 120  0.1472 0.1088 12.92 0.2322 0.6605 50.29

[0085] TABLE-1 shows that luminance decreases as the content of the phosphor increases. However, the values of x, y increase as the content of the phosphor, which shows emission of a reddish light.

[0086] [Relationship Between the Content of the Red Phosphor and the Emission Color of the LED Device Where a Plurality of Kinds of YAG Phosphors Exist]

[0087] Compositions for the covering members were prepared by adding 20 parts by mass of Y_(1.8)Gd_(1.2)Al₅O₁₂ in which 4 mol % of Ce³⁺ ions are added to a host crystal and quantities of LiEu_(0.7)Y_(0.3)W₂O₈ as shown in TABLE-2 with respect to 100 parts by mass of silicone rubber. Cap-shaped covering members having a thickness of 0.35 mm were formed using these compositions.

[0088] The thus formed covering members were mounted on cannonball-shaped sealed members in which blue light-emitting elements (main emission wavelength of 468 nm) are embedded to produce white LED devices, and the emission spectra and the emission characteristics of the white LED devices were measured. The emission characteristics are shown in TABLE-2 and the emission spectra are shown in FIG. 10. It should be noted that emission intensity in FIG. 10 is a relative value on the assumption that emission intensity at 468 nm is 1 in the case that the content of the red phosphor is 0. TABLE 2 Content of LiEu_(0,7)Y_(0,3)W₂O₈ Blue LED Chip (parts by mass) x y Luminance  0 0.3125 0.3381 54.25 10 0.3571 0.3909 51.79 20 0.3665 0.3981 46.39 30 0.3753 0.4039 43.77 40 0.3731 0.3971 40.17 50 0.3677 0.3868 36.01 60 0.3646 0.3810 35.71

[0089] As can be seen from TABLE-2, chromaticity shifts from white toward yellow up to the LiEu_(0.7)Y_(0.3)W₂O₈ content of 30 parts, but conversely shifts toward white as the content exceeds 40 parts. Further, luminance is found to decrease as the LiEu_(0.7)Y_(0.3)W₂O₈ content increase.

[0090] It can be seen from FIG. 10 that emission intensity at 468 nm is reduced by adding LiEu_(0.7)Y_(0.3)W₂O₈ and an emission peak of LiEu_(0.7)Y_(0.3)W₂O₈ by ⁵D₀→⁷F₂ transition is located at 616 nm. As the content of LiEu_(0.7)Y_(0.3)W₂O₈ increases, emission intensity at 468 nm decreases and emission intensity tends to decrease as a whole, but emission intensity at 616 nm increases. Specifically, a ratio (I_(a):I_(b)) of emission intensity (I_(a)) at 468 nm to emission intensity (I_(b)) at 616 nm was 1:0.5 to 1:2.

[0091] [Color Rendering Property of the LED Device]

[0092] LED Devices 1 and 2

[0093] Resin compositions for the covering members were prepared by adding a YAG phosphor in quantities shown in TABLE-3 to 100 parts by mass of silicone rubber as a base polymer, and covering members No. 1 and 2 having a thickness of 0.35 mm were molded. These covering members were mounted on sealed members in which blue light-emitting elements are embedded, and the color rendering property and emission characteristics of the thus formed LED devices 1 and 2 were measured. A measurement result is shown in TABLE-3.

[0094] A red YAG phosphor was used as the YAG phosphor.

[0095] LED devices 3 to 5

[0096] Resin compositions for the covering members were prepared by adding a YAG phosphor and CaS:Eu as a red phosphor in quantities shown in TABLE-3 to 100 parts by mass of base polymer (silicone rubber), and covering members No. 3 to 5 having a thickness of 0.35 mm were molded. These covering members were mounted on sealed elements in which blue light-emitting elements are embedded, and the color rendering property and emission characteristics of the thus formed LED devices 3 to 5 were measured. A measurement result is shown in TABLE-3. Further, emission spectra of the LED devices 4 and 5 are shown in FIGS. 11 and 12.

[0097] A red YAG phosphor or green YAG phosphor was used as the YAG phosphor.

[0098] LED Device 6

[0099] Resin compositions for the covering member were prepared by adding a YAG phosphor and azo pigment as a red pigment in quantities shown in TABLE-3 to 100 parts by mass of base polymer (silicone rubber), and a covering member No. 6 having a thickness of 0.35 mm were formed. This covering member was mounted on a sealed member in which a blue light-emitting element is embedded, and the color rendering property and emission characteristics of the thus formed LED device 6 were measured. A measurement result is shown in TABLE-3. Further, emission spectrum of the LED 6 is shown in FIG. 13.

[0100] LED Device 7

[0101] LED device 7 is a commercially available white LED device in which a fluorescent layer containing YAG is placed on a blue light-emitting element and these two as a whole are sealed with a resin to form a cannonball shape. It should be noted that the kind of the blue light-emitting element used in the LED device 7 is different from the one used in the white LED devices 1 to 6.

[0102] A measurement result of the color rendering property and emission characteristic of this white LED device is shown in TABLE-3, and an emission spectrum thereof is shown in FIG. 14.

[0103] It should be noted that vertical axis in FIGS. 11 to 14 represents a relative emission intensity when the emission intensity of blue (FIGS. 11, 13, 14) or red (FIG. 12) light at an emission peak is assumed to be 100. TABLE 3 NO. 1 2 3 4 5 6 7 Composition (Parts by Mass) Base Polymer 100 100 100 100 100 100 Commercially Red YAG 14.2 19.7 19.4 — — — available Green YAG — — — 8.8 10.8 12.8 product CaS:Eu — — 4.3 8.9 11.0 — Red Pigment — — — — — 0.1 — Evaluation Ra 75.4 83.6 87.3 85.2 88.9 72.4 74.9 R₉ 87 45 71 96 96 88 85 R₁₀ 55 93 91 74 85 44 63 R₁₁ 66 69 75 81 83 71 59 R₁₂ 61 74 74 77 80 54 57 R₁₃ 84 95 98 93 95 80 87 R₁₄ 85 92 94 89 94 81 87 R₁₅ 82 87 93 90 90 84 80 Luminance L 68.19 71.90 66.68 63.64 65.18 49.71 111.90 Chromaticity Coordinate x 0.3368 0.3678 0.3900 0.3486 0.3848 0.3394 0.3285 Chromaticity Coordinate y 0.3326 0.3655 0.3665 0.3646 0.3904 0.3255 0.3166

[0104] In FIG. 13, a peak, which shows emission intensity of about 60% of the blue light, is seen at 580 to 600 nm in addition to the one at 458 nm which is the wavelength of the light from the light-emitting element and an amount of light is reduced near 500 nm since the red YAG is used.

[0105]FIG. 14 shows an emission spectrum in the case that a red pigment was contained. Although a spectrum pattern thereof is similar to the one shown in FIG. 13, emission intensity is reduced as a whole in the case that the pigment was used as can been seen from the fact that intensity is 100% at 3.3×10⁻³ in FIG. 13 while being at 1.62×10⁻³ in FIG. 14.

[0106] On the other hand, FIGS. 11 and 12 show emission spectra of the LED device according to the invention, in which a peak is seen near 650 nm and intensity at this peak is 60% or more of that at the peak of the blue light. Further, a fall in emission near 500 nm as shown in FIG. 13, 14 was not recognized. In other words, average emission is made in a range between 400 nm and 700 nm, indicating an improvement in color rendering property.

[0107] Such an improvement in color rendering property is also clear from TABLE-3 since only the inventive white LED devices containing the specific red phosphors have an average color rendering evaluation number Ra of 85 or larger.

[0108] Specifically, even if the content of the red YAG phosphor is increased as in the LED device 2, an average color rendering evaluation number Ra of 85 or larger cannot be attained, and a yellow light emitted from YAG becomes excessively strong, thereby significantly reducing the special color rendering evaluation number R₉ which is an index for distinct red. On the other hand, the inventive white LED devices attained an average color rendering evaluation number Ra of 85 or larger independently of the kind of the YAG phosphor and presented an excellent color rendering property without significantly reducing the values of the special color rendering evaluation numbers R₉ to R₁₅. Specifically, the inventive white LED devices had better color rendering property than any of the LED devices 1 and 2 in which the red YAG was used, the LED device 6 in which the red pigment was used, and the commercially available LED device 7. Since the red pigment was added in the covering member of the LED device 6, the special color rendering evaluation number R₁₀ which is an index for yellow was significantly reduced and luminance was reduced due to an influence of the pigment although the value of the special color rendering evaluation number R₉ which is an index for distinct red was increased.

[0109] Although luminance is particularly higher in the commercially available LED device 7, this is thought to be based on a difference between the embedded blue light-emitting elements.

[0110] As described above, the inventive light-emitting diode has a better red emission than the conventional light-emitting diodes using a light-emitting element for emitting light of blue to green region. Accordingly, the inventive white LED device in which YAG phosphor is added in the fluorescent substance has a color rendering property approximate to that of a fluorescent lamp or natural light. Further, if the inventive covering member is mounted on a conventional light-emitting diode device, the inventive white LED devices having an excellent color rendering property can be obtained.

[0111] This application is based on Japanese Application No. 2000-70878 filed in Japanese Patent Office on Mar. 14, 2000, the contents of which are hereby incorporated by reference.

[0112] Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein. 

What is claimed is:
 1. A light-emitting diode device, comprising a light-emitting element for emitting light of blue to green region and a fluorescent substance containing a CaS phosphor activated by Eu for converting a wavelength of the light emitted from the light-emitting element.
 2. The light-emitting diode device according to claim 1 , wherein the light-emitting element emits light having a wavelength of 400 nm to 600 nm.
 3. The light-emitting diode device according to claim 1 , wherein the fluorescent substance further contains a Y_(x)Gd_(3-x)Al₅O₁₂ (0≦x≦3) phosphor activated by Ce.
 4. The light-emitting diode device according to claim 3 , wherein the light-emitting diode device has an average color rendering evaluation number (Ra) of 85 or larger.
 5. The light-emitting diode device according to claim 3 , wherein the light-emitting diode device has a special color rendering evaluation number R₉ of 80 or larger.
 6. The light-emitting diode device according to claim 2 , wherein a ratio (I_(a):I_(b)) of emission intensity (I_(a)) at 470 nm±2 nm to emission intensity (I_(b)) at 610 to 640 nm is 1:0.5 to 1:2.
 7. The light-emitting diode device according to claim 1 , wherein the fluorescent substance is in the form of a layer placed on an outer surface of the light-emitting element.
 8. The light-emitting diode device according to claim 1 , wherein the fluorescent substance is made of a transparent resin containing the CaS phosphor and seals the light-emitting element.
 9. The light-emitting diode device according to claim 1 , wherein the light-emitting element is sealed with a transparent resin to form a sealed member, the sealed member is covered with a covering member including the fluorescent substance.
 10. The light-emitting diode device according to claim 9 , wherein the covering member is made from a polymer composition containing 100 parts by mass of a base polymer, 2 to 40 parts by mass of a phosphor expressed by AEu_((1-x))Ln_(x)B₂O₈, and a Y_(x)Gd_(3-x)Al₅O₁₂ phosphor activated by Ce.
 11. The light-emitting diode device according to claim 10, wherein the base polymer comprises a rubber and/or a thermoplastic elastomer.
 12. The light-emitting diode device according to claim 9 , wherein the covering member is made from a polymer composition containing 100 parts by mass of a base polymer, 2 to 40 parts by mass of CaS activated by Eu, and a Y_(x)Gd_(3-x)Al₅O₁₂ phosphor activated by Ce.
 13. The light-emitting diode device according to claim 12 , wherein the base polymer comprises a rubber and/or a thermoplastic elastomer.
 14. A light-emitting diode device, comprising a light-emitting element for emitting light of blue to green region and a fluorescent substance containing a phosphor expressed by a general formula AEu_((1-x))Ln_(x)B₂O₈ for converting a wavelength of the light emitted from the light-emitting element, wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo; and x is number equal to or larger than 0, but smaller than
 1. 15. The light-emitting diode device according to claim 14, wherein the light-emitting element emits light having a wavelength of 400 nm to 600 nm.
 16. The light-emitting diode device according to claim 14 , wherein the fluorescent substance further contains a Y_(x)Gd_(3-x)Al₅O₁₂ (0≦x≦3) phosphor activated by Ce.
 17. The light-emitting diode device according to claim 16 , wherein the light-emitting diode device has an average color rendering evaluation number (Ra) of 85 or larger.
 18. The light-emitting diode device according to claim 16 , wherein the light-emitting diode device has a special color rendering evaluation number R₉ of 80 or larger.
 19. The light-emitting diode device according to claim 15 , wherein a ratio (I_(a):I_(b)) of emission intensity (I_(a)) of light having 470 nm±2 nm to emission intensity (I_(b)) of light having 610 to 640 nm is 1:0.5 to 1:2.
 20. The light-emitting diode device according to claim 14 , wherein the fluorescent substance is in the form of a layer placed on an outer surface of the light-emitting element.
 21. The light-emitting diode device according to claim 14, wherein the fluorescent substance is made of a transparent resin containing the AEu_((1-x))Ln_(x)B₂O₈ phosphor and seals the light-emitting element.
 22. The light-emitting diode device according to claim 14 , wherein the light-emitting element is sealed with a transparent resin to form a sealed member, the sealed member is covered with a covering member including the fluorescent substance.
 23. The light-emitting diode device according to claim 22 , wherein the covering member is made from a polymer composition containing 100 parts by mass of a base polymer, 2 to 40 parts by mass of a phosphor expressed by AEu_((1-x))Ln_(x)B₂O₈, and a Y_(x)Gd_(3-x)Al₅O₁₂ phosphor activated by Ce.
 24. The light-emitting diode device according to claim 23 , wherein the base polymer comprises a rubber and/or a thermoplastic elastomer.
 25. The light-emitting diode device according to claim 22 , wherein the covering member is made from a polymer composition containing 100 parts by mass of a base polymer, 2 to 40 parts by mass of CaS activated by Eu, and a Y_(x)Gd_(3-x)Al₅O₁₂ phosphor activated by Ce.
 26. The light-emitting diode device according to claim 25 , wherein the base polymer comprises a rubber and/or a thermoplastic elastomer.
 27. A covering member for covering a light-emitting diode device which includes a light-emitting element sealed by a transparent resin for emitting light of blue to green region, comprising a base polymer, and a CaS phosphor activated by Eu.
 28. The covering member according to claim 27 , further comprising a Y_(x)Gd_(3-x)Al₅O₁₂ phosphor activated by Ce.
 29. A covering member for covering a light-emitting diode device which includes a light-emitting element sealed by a transparent resin for emitting light of blue to green region, comprising a base polymer, and a phosphor expressed by a general formula AEu_((1-x))Ln_(x)B₂O₈ wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo; and x is number equal to or larger than 0, but smaller than
 1. 30. The covering member according to claim 29 , further comprising a Y_(x)Gd_(3-x)Al₅O₁₂ phosphor activated by Ce. 